Update code for ES6 syntax.
Update code for ES6 syntax.

var -> const, let.
Correct linter errors.
Add `//jshint ignore: line` to suppress linter messages
about `EventSource`

ga-wdi-boston/rails-tic-tac-toe-api,ga-wdi-boston/rails-tic-tac-toe-api,ga-wdi-boston/rails-tic-tac-toe-api